So much depends on your circumstances.
If you have several local shows, and train/ handle the dog yourself, it doesn't cost very much.
If you have a quality dog, s/he will finish much faster, than a dog that has to be entered enough times to find a blind judge.
If your own regional club sponsors shows that are politically heated, its a waste of time to show there.
If you have chosen a breed that is hard to find a major for ( enough entries of that breed) it can get expensive.
If you have chosen a breed that requires professional grooming, show grooming can be very expensive, whereas a wash n wear breed doesn't doesn't require a pro.
If you are careful to train correctly, ( from the VERY beginning) and train yourself as an able handler, it will cost less than having to hire it done. Even the best dog can't win with an inept handler or poor training.
I'm lucky enough to live where there are several cluster shows within a 3 hour drive. My dogs are good enough to take BOB in nearly every show, so that I can finish a dog in a season, with very little travel expense.
I just returned here from a small coastal community. If I had chosen to campaign a dog while living there, I would have been out a LOT more..as each show would have required hours of travel, overnight housing, and a 7 hour "mountainous" drive to find a major in my breed.
I looked into sending a dog out with a handler..It would have cost $500 per month plus his other expenses.
So, the expenses depend on many factors.
I only have to put out about $200-300 in club membership and entry fees, and about $200 travel costs.
You should contact your local kennel club, and ask..They will be happy to share their knowledge. Better yet, join your local club, as there are a LOT of perks, including training classes.
You can find your local club in the akc.org website.
Important***
Take a mentor with you to choose the puppy. There is no substitute for the experienced eye.
Added:
I have tried co-ownerships. They are too confining, and don't allow leeway for life's happenings. The dog may be taken from you if you become ill and can't show, or have to move from the region..etc..I would never co-own again, and don't recommend it.
Now, if the breeder would want to just mentor you, that works..